Tuesday, July 23, 2013

Java 1.5 introduced Thread pool in Java in form of Executor framework,
which allows Java programmer to decouple submission of task to execution of
task. If you are doing server side programming in Java than Thread pool is an
important concept to maintain scalability, robustness and stability of system.
For those, who are not familiar with thread pool in Java or concept of thread
pool here is one liner, Thread pool in Java is pool of worker threads, which is
ready to perform any task given to them, mostly in form of implementation of Runnable
or Callable
interface. Since Java supports multithreading in programming language itself, it
allows multiple thread to run concurrently and perform parallel processing of
task. In this article we will learn following things about thread pool in Java
:

What is Thread pool in Java?

Why do we need Thread pool in Java ?

What is Executor framework in Java 5?

How to create fixed size thread pool using Executor framework in Java?

Benefits of using Thread Pool in Java?

What is Thread Pool in Java and why we need
it

As I said Thread pool is pool of already created worker thread ready to
do the job. Thread pool is one of essential facility any multi-threaded server
side Java application requires. One example of using thread pool is creating a web
server, which process client request. If you are familiar with socket
programming than you know that ServerSocket.accept() is blocking
method and blocks until a socket connection made. If only one thread is
used to process client request, than it subsequently limit how many client can
access server concurrently. In order to support large number of clients, you
may decide to use one thread per request paradigm, in which each request is
processed by separate Thread, but this require Thread to be created, when
request arrived. Since creation of
Thread is time consuming process, it delays request processing. It also limits
number of clients based upon how many thread per JVM
is allowed, which is obviously a limited number. Thread pool solves this
problem for you, It creates Thread and manage them. Instead of creating Thread
and discarding them once task is done, thread-pool reuses threads in form of
worker thread. Since Thread are usually created and pooled when application
starts, your server can immediately start request processing, which can further
improve server’s response time. Apart from this, there are several other
benefits of using Thread pool in Java applications, which we will see in subsequent
section. In short, we need thread pools to better mange threads and decoupling
task submission from execution. Thread pool and Executor framework
introduced in Java 5 is an excellent thread pool provided by library.

Java Thread Pool - Executor Framework in
Java 5

Java 5 introduced several useful features like Enum,
Generics,
Variable
arguments and several concurrency collections and utilities like ConcurrentHashMap and BlockingQueue
etc, It also introduced a full feature built-in Thread Pool framework commonly
known as Executor framework. Core of this thread pool framework is Executor interface
which defines abstraction of task execution with method execute(Runnable
task) and ExecutorService which extends Executor to add
various life-cycle and thread pool management facilities like shutting down
thread pool. Executor framework also provides an static
utility class called Executors ( similar to Collections) which
provides several static factory method to create various type of Thread Pool
implementation in Java e.g. fixed size thread pool, cached thread pool and
scheduled thread pool. Runnable and Callable interface
are used to represent task executed by worker thread managed in these Thread
pools. Interesting point about Executor framework is that, it is based on Producer
consumer design pattern, where application thread produces task and worker
thread consumers or execute those task, So it also suffers with limitation of
Producer consumer task like if production speed is substantially higher than
consumption than you may run OutOfMemory because of
queued task, of course only if your queue is unbounded.

How to create fixed size thread pool using
Executor framework in Java?

Creating fixed size thread pool using Java 5 Executor framework is pretty
easy because of static factory methodsprovided by Executors class. All you need to do is
define your task which you want to execute concurrently and than submit that
task to ExecutorService. from them Thread pool will take
care of how to execute that task, it can be executed by any free worker thread
and if you are interested in result you can query Future object
returned by submit() method. Executor framework also
provides different kind of Thread Pool e.g. SingleThreadExecutor which
creates just one worker thread or CachedThreadPool which
creates worker threads as and when necessary. You can also check Java documentation of Executor Framework for
complete details of services provided by this API. Java concurrency in Practice
also has couple of chapters dedicated to effective use of Java 5 Executor
framework, which is worth reading for any senior Java developer.

Example of Thread Pool in Java

Here is an example of Thread pool
in Java, which uses Executor framework of Java 5 to create a fixed thread
pool with number of worker thread as 10. It will then create task and submit
that to Thread pool for execution:

If you look at output of this Java example you will find different
threads from thread pool are executing tasks.

Benefits of Thread Pool in Java

Thread Pool offers several benefit to Java application, biggest of them
is separating submission of task to execution of task ,which result if more
loose coupled and flexible design than tightly coupled create and execute
pattern. Here are some more benefits of using Thread pool in Java:

1) Use of Thread Pool reduces response time by avoiding thread creation
during request or task processing.

2) Use of Thread Pool allows you to change your execution policy as you
need. you can go from single
thread to multiple thread by just replacing ExecutorService
implementation.

3) Thread Pool in Java application increases stability of system by
creating a configured number of threads decided based on system load and available
resource.

That's all on Thread pool in Java 5. we have seen what is thread pool in
Java, what is executor framework in java 5, how to create thread pool in Java
and some benefits of using thread pool in Java application. no doubt knowledge
of thread pool is essential for a server side core Java developer and I suggest
reading Java Threads and Concurrency Practice in Java
to learn more about concurrency and thread pool.

Recommended Books in this article

Java Concurrency in Practice by Brian Goeatz, Doug Leaa, Joshua Bloch and team